4210 brake problem

I am working on a 4210-it has little or no braking on the right side wheel. About a year and a half ago,I replaced both rear brake piston o ring sets.Still no/weak right side brake.Went ahead and replaced the master cylinder.Same problem. Tractor had to go as it was needed.Got the tractor back,pressure tested at the axle and it held 100 psi for 5 days. I replaced master cylinder again.Same problem.Since working pressure is around 400psi, I went ahead and replaced piston o rings again,including the one behind the bearing retainer. When I bleed it,I get maybe 2 pumps of solid fluid,then 1 that is aerated.The pedal never gets more than an inch above the foot rest of semi solid pedal.Due to sometimes aerated fluid,and that the old reservoir only had 3/8 inch of fluid,I replaced it,thinking the port to the master cylinder was not covered completely at times. New redesigned reservoir stays full. I checked the fluid going into the reservoir and it it is ok - not aerated. The pedal is making a complete stroke into master cylinder.The line from the master cylinder to the axle is not leaking. So the gist of my problem is a short reaction stroke of brake (the low pedal) and every third stroke of the pedal, has aerated fluid after a few solid pumps at the bleeder. I would suspect the small o ring under the transmission cover that leads to the bleeder,except it holds the 100psi pressure.It acts like,in my mind,that master cylinder is trying to fill a bigger void than just the area behind the piston.Left brake is fine-solid as can be.I have worked on about a dozen, or more of these style brakes,but this one about has me stumped!Any ideas,suggestions,or help is greatly appreciated.Thanks ,Mark.

Hi Mark, Can you swap the brake lines at the master cylinder so left brake goes to right wheel and right brake to left wheel?
That would determine if problem is in master cylinder or beyond.
Just a hint, I always had better luck bleeding the brakes on those UK tractors just by warming up the Hytran and run engine at about 1500 RPM and then open one bleeder screw at a time and with a clear vinyl tube connected to the bleeder screw let it flow into a clean bucket until all the air bubbles are gone. I never pump the brakes.

JimB

I have bled them both ways,some tractors are easier than others.But this one is kicking my butt.I thought of switching the lines,but after 3 master cylinders, I don't think it will change anything.Mark.

Make sure the primary cup in the MC is pulling back far enough to clear the compensating port. Jim